Polyesters that are semicrystalline, particularly poly(butylene terephthalate) and poly(ethylene terephthalate), are used extensively in many applications that require good solvent resistance and good properties at elevated temperatures. They are ordinarily processed by injection molding, but there are many components of automobiles and other systems wherein such parts are hollow and to manufacture these by injection molding is very difficult and expensive. Many such parts can conceivably be made by blow molding provided the polymer system has adequate melt strength and viscosity. Unfortunately, polyesters commonly used for injection molding have melt viscosities which are too low to make them suitable for extrusion blow molding. High molecular weight polyesters can be made by solid phase polymerization of polymers suitable for injection molding, but this operation raises the cost of the polyesters substantially. It would be desirable to have blow moldable polyester compositions made from the commercial injection moldable grades of polyesters.
For many applications the inherent rigidity of semicrystalline polyesters is important. The addition of conventional di- or poly-epoxides and, more recently, the addition of ethylene copolymers containing glycidyl groups have been suggested for increasing the melt strength and viscosity of polyesters (see Kometani et al., U.S. Pat. No. 4,246,378). These solutions to the problem have improved polyesters for certain blow molding applications but have proved to be inadequate in providing materials suitable for blow molding large objects having complex cross-sections such as automobile parts. Another problem encountered in the use of the epoxide-containing additives is their effect on modulus or rigidity of the final molded parts. The amount of epoxide-containing additives required to significantly increase melt strength at the same time reduces rigidity. This is particularly true for the ethylene copolymers containing glycidyl groups. While these copolymers are generally effective for increasing melt strength they do lead to a substantial loss of rigidity because of their rubbery nature. Therefore, a need still exists for readily blow moldable polyester compositions which retain the high modulus of unmodified polyesters to a greater degree.